The present invention relates generally to gas turbine engines, and, more specifically, to turbine blades therein.
In a gas turbine engine air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Turbine stages extract energy from the combustion gases for powering the compressor and producing useful work.
A high pressure turbine (HPT) immediately follows the combustor and includes one or more rows of turbine rotor blades which extract energy from the gases for powering the compressor.
A multistage low pressure turbine (LPT) follows the HPT and includes additional rotor blades which also extract energy from the combustion gases for typically powering an upstream fan in a turbofan aircraft engine application.
In yet another configuration, an intermediate pressure turbine (IPT) is disposed between the HPT and the LPT and includes additional turbine stages which also extract energy from the combustion gases for producing additional work. In this configuration, the IPT may be joined to a low pressure or booster compressor disposed upstream from the high pressure compressor for providing power thereto. And, the LPT powers an external drive shaft which may be used for powering an electrical generator in a typical industrial gas turbine engine application.
Since the rotor blades of the HPT are subject to the hottest temperatures of the combustion gases and rotate at high speed they are subject to considerable thermal and centrifugal stresses during operation. In order to improve their life and durability, the HPT turbine blades are typically formed of superalloys, such as nickel-based metal, for their enhanced strength at elevated temperature.
The turbine blades typically include hollow airfoils having cooling circuits therein through which is circulated cooling air bled from the compressor during operation. The blades also include an integral platform which defines the inner boundary for the hot combustion gases, with an integral supporting dovetail being disposed therebelow.
The blade dovetail includes one or more pairs of dovetail tangs or lobes which are mounted in corresponding dovetail slots in the perimeter of the supporting turbine rotor disk. Axial-entry dovetails are common and extend through axial dovetail slots disposed around the perimeter of the rotor disk.
Centrifugal loads generated during operation of the rotating blades are carried radially inwardly through the dovetail lobes into the corresponding disk posts which define the dovetail slots around the perimeter of the rotor disk. Since the blade airfoil, platform, and dovetail are specifically configured for different purposes they experience different thermal and centrifugal loads during operation which must be suitably limited for ensuring a long useful life of the turbine stage.
Correspondingly, the supporting rotor disks must also be suitably configured for carrying the centrifugal loads from the row of rotor blades with limited stress for ensuring the long useful life thereof as well.
Since the blade dovetails are discrete components spaced apart around the perimeter of the rotor disk and separated from each other by the intervening disk posts, corresponding inter-blade spaces or voids are created which must be suitably sealed during operation. Hot combustion gases flow between the blades during operation outside the blade platforms. And, cooling air bled from the compressor is channeled through the disk slots and through cooling channels extending radially through the individual rotor blades.
Accordingly, individual seal bodies are typically configured to fill the inter-blade voids between adjacent dovetails and above the corresponding disk posts therebetween. The seal bodies limit the leakage between the forward and aft faces of the blades when mounted to the perimeter of the rotor disk. These seals also control the operating temperature of the disk posts for maximizing the useful life of the rotor disk.
Turbine efficiency is affected by many interrelated parameters. Fundamentally, turbine efficiency may be increased as combustion gas temperature increases, but hotter combustion gases increase the heat loads on the turbine components which must be suitably cooled. Bleeding cooling air from the compressor in turn reduces efficiency of the engine since the bleed air is not used in the combustion process.
Furthermore, the aerodynamic profiles of the turbine blades themselves affect engine efficiency, and the airfoil configuration also affects thermal and centrifugal loads and stresses not only in the airfoils themselves but also in the supporting platforms, dovetails, and rotor disk posts.
In a recent development of a three-spool industrial gas turbine engine having an HPT, IPT, and LPT for driving an electrical generator, engine efficiency is being increased by improving the 3-D aerodynamic configuration of the turbine airfoils in the second stage of the HPT, for example. Modem analysis tools are being used to refine the 3-D configuration of the second stage airfoil for improving the efficiency thereof, which blades experience a larger twist relative to the axial dovetails than conventionally found. Such axial dovetails have proven durability in conventional turbines and permit corresponding long life of the supporting rotor disk.
The increased twist or turning of the airfoil root or hub at the inner platform correspondingly changes the load paths to the axial dovetail which is not similarly twisted or turned about the radial axis.
The airfoil hub turning requires suitably inclined or angled axial splitlines between the adjacent platforms in the row of turbine blades. The blade platforms therefore are twisted relative to the supporting axial dovetail and affect the centrifugal load path therethrough as well as the collective configuration thereof.
Since both the axial dovetail and inter-blade seal body are axisymmetrical the inter-blade spacing is affected by the 3-D airfoil and twisted platform which can lead to undesirable flow leakage between the blades.
Accordingly, it is desired to provide an improved turbine blade having a twisted platform joined to an axial dovetail with features for reducing inter-blade leakage.